It is difficult to measure a micro-signal of a micro-current or the like. This is because a signal to be measured, which is extremely small, is subject to influence of extrinsic noise. In other words, the signal is subject to the influence of noise due to a low signal-to-noise (S/N) ratio at a time of measurement. In addition, it is known that the influence of noise can be suppressed by restricting a frequency bandwidth by increasing an integration time (aperture time) for a measuring instrument when the signal to be measured is a direct current or when a frequency component thereof is sufficiently low. However, the measurement time becomes longer as the integration time is increased due to a trade-off therebetween, which raises a problem in that it is necessary to appropriately select the integration time.
To measure a signal on which a given amount of noise is superposed, an observer needs to adjust the integration time while checking a measurement result because it is difficult to know what length of time is appropriate as the integration time. Therefore, it is necessary to undertake a process of trial and error until a desired or optimum integration time is found. This work may require expert skills, and it may be difficult for the untrained.
For example, U.S. Pat. No. 6,559,782 discusses that the integration time may be set in terms of a multiple of a power line cycle (PLC).
Also, with reference to FIG. 10 herein, the setting of an integration time by taking a screen display for measurement results of a known digital multimeter, as an example, will be described. FIG. 10 illustrates an indication 1002 of a direct current measurement using the known multimeter, in which “+80.034 nA” or “+80.035 nA” is displayed to indicate real-time measurement results, and a least significant digit 1004 of a measured value whose indication exhibits variations, in which “4” and “5” are alternately displayed. The variations may be extended to the least significant two or three digits depending on the relationship between a signal to be measured and the integration time.
The above-mentioned case is interpreted to be a state in which the indication varies due to a short integration time, and the user may use Number of Power Line Cycles (NPLC) command or an aperture (APER) command to set an appropriate integration time so that the variations in the indication fall within a desired range.
However, in regard to an optimum range of the variations in the indication, the indication illustrated in FIG. 10 is not intuitive, which causes a difference in selection of the integration time to be set depending on the user's level of skill.
There is also known an apparatus disclosed in U.S. Pat. No. 6,911,831 for automatically switching a measuring range to an appropriate one in accordance with a magnitude of the signal to be measured which contains noise. However, there is no apparatus for extracting a noise component and presenting the user with an index of an appropriate integration time.
It may be desired to provide information useful for selection of an appropriate measurement condition (integration time) at a time of measuring a signal having a low S/N ratio such as a micro-current measurement.